Conventional self-oscillating sweep generators of the type used for deflection circuit applications in television receivers employ a condenser chargeable from a power supply over a resistance, and a voltage divider connected across the supply. A reference voltage derived from the output of the voltage divider is connected to one input of a normally disabled threshold comparator circuit constructed, e.g. from two complementary transistors. The voltage across the condenser is coupled to the other input of the comparator circuit. The comparator circuit is effectively disposed in shunt with the condenser so that when the reference voltage is exceeded by the condenser voltage during the charging thereof, a discharge path is provided for the condenser to terminate the sweep and immediately thereafter to restore the comparator circuit to its high impedance condition.
When such types of sweep generators are employed as vertical oscillators in T.V. receivers and the like, facilities must be provided for varying the frequency of the sweep to permit synchronization. In general, such variation of the frequency is accomplished by altering the resistance ratio of the voltage divider as the syncrate varies to corresponding modify the reference voltage. This results in establishing the point during the sweep at which the capacitor-shunting comparator circuit is switched into its low impedance condition. One problem with conventional sweep generators of this type is that the above-mentioned changes in frequency can be effected not only by the normal repetitive sync pulses, but also by disturbance pulses occurring between successive sync pulses. When the sweep circuit responds to such disturbance pulses, the stability and/or size of the T.V. picture can be adversely affected.